Triply-Resonant Quantum Transducer for Quantum Computing
Quantum Transducer
Stanford University and SLAC National Accelerator Laboratory scientists developed a mm-wave-to-optical converter for quantum communications and next-generation computing. A triply-resonant integrated superconducting electro-optic transducer was disclosed in Nature Communications. Future quantum processor networking may require this component.
The Millimeter Wave Frontier
Modern communication systems must operate at higher frequencies to provide faster data transfer rates and visual resolution. The mm-wave spectrum, from 30 GHz to 300 GHz, is essentially unexplored for classical and quantum applications, while common microwave systems operate in the low gigahertz (GHz) region.
The researchers focused on 107 GHz under Jason F. Herrmann and Kevin K. S. Multani. This high-frequency regime is useful for quantum physics because superconducting processors can function at higher temperatures than microwave qubits. More cooling power may help fault-tolerant quantum computing overcome its massive scalability challenges.
Architecture of Transducer
The device relies on complicated light-matter interactions. The transducer uses an on-chip niobium titanium nitride (NbTiN) superconducting resonator and a thin-film lithium niobate (TFLN) optical racetrack resonator.
Functioning requires the electro-optic effect. A mm-wave signal at 107 GHz modulates the X-cut TFLN's refractive index. This modulation creates new light sidebands with a telecom wavelength “pump” laser. The researchers created a triply-resonant system by matching the mm-wave mode frequency to the optical cavity's free spectral range (FSR). In this system, the generated optical sideband, optical pump, and input mm-wave are all resonant for optimal photon conversion.
Researchers found an average single-photon interaction rate (g0/2π) of 0.7 kHz and a maximum photon transduction efficiency of ηOE≈0.82×10−6.
Overcoming Substrate Interference
One major concern in the inquiry was “substrate modes” interference. A 500 µm thick sapphire substrate acts as the device's dielectric cavity at mm-wave frequencies. The superconducting resonator may hybridize with these extraneous modes, leaching photons and decreasing device performance.
The researchers developed a complex input-output model with various parameters to simulate the transmission spectrum. According to their calculations, lowering the sapphire chip's physical volume might reduce the number of substrate modes from 29 to one in the 95 GHz to 110 GHz region. This would enable hardware upgrades.
“Quasiparticle” Challenge
Another challenge is superconductor-laser interaction. The superconducting electrodes absorb stray photons as the optical pump intensifies. It splits Cooper pairs, electron pairs that give resistance-free current, and creates non-equilibrium quasiparticles.
These quasiparticles enhance the cavity's temperature by increasing internal loss and dissipation. By evaluating this impact, scientists discovered a 0.86 µK temperature shift for each additional intracavity photon. Pulsing the optical pump between conversion events allows the quasiparticle density to return to thermal equilibrium, which could reduce this in future quantum applications.
The Path to Quantum Networks
Despite these challenges, integrated mm-wave photonic systems may be superior for hybrid quantum systems, according to the experiment. Once built, this transducer might connect superconducting qubits to optical fibers to send quantum information across long distances at normal temperature.
These transducers can operate at 5 K, unlike millikelvin microwave qubits, proving they can work at greater temperatures in a dilution refrigerator. This may lead to multistage transduction, which connects microwave gear to optical networks via millimeter-wave intermediaries.
Researchers say this finding adds a vital “missing element” to the quantum toolbox: an integrated technique to translate light's long-distance capabilities and millimeter-waves' high-speed world.